Experimental - Cancel-able Menus

[kantenkugel]

Pull Request

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• [x] I have checked the pull request page for upcoming or merged features/bug fixes.
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Pull Request Information

Check and fill in the blanks for all that apply:

• [ ] My PR fixes a bug, error, or other issue with the library's codebase.
• [x] My PR is for the menu and commons modules of the JDA-Utilities library.
• [ ] My PR creates a new module for the JDA-Utilities library: ______.

Description

This module adds the possibility to cancel EventWaiter tasks via the new return value of EventWaiter#waitForEvent (Future<Void>)

This new feature is used to add Menu#cancel() to cancel menus that are already running.

Todo:

• [ ] Discuss if this is the proper way of doing this or if some mechanics should be changed
• [x] Javadocs
• [x] Decide how to handle cancelFuture.cancel(true) vs false
• [ ] Should there be a cancel method that doesn't execute the cancel/final action? What about the EventWaiter cancellation?
• [ ] Eventually throw errors on display() if already being displayed somewhere, as this would currently prevent proper functionality of Menu#cancel()
• [ ] Wait for PR #58 to be merged and patch it as well in this PR
• [ ] Extensive testing
• [ ] Final review

[kantenkugel]

Should probably also wait for #58 to be merged so i can apply the changes from this PR to that code as well. Adding it to the TODO